Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a low frequency alternating current. In a switching power supply an alternating current (ac) input is converted to provide a well regulated direct current (dc) output through high frequency (HF) switching and an energy transfer element, for example, an inductor or a transformer. The switched mode power supply controller usually provides output regulation by sensing the output and controlling it in a closed loop. In operation of a switched mode power supply the high frequency switching is utilized to provide the desired output by varying the on-time, off-time or switching frequency of the switch. Typically, in pulse width modulation PWM, duty cycle is controlled, where the duty cycle is the ratio of the on time to the total switching period.
Requirements, such as efficiency, size, weight and cost are usually taken into account when designing a switched mode power supply. A controller that controls the switching of the switched mode power supply may be designed to comply with efficiency requirements of certain regulatory agencies. For example, a conventional controller may be designed to control the switching of the switched mode power supply to reduce power consumption at no load or low load power conditions while at the same time be designed to increase efficiency at higher load conditions.
One method for reducing power loss at low load and no load conditions of the switched mode power supply is commonly known as “Burst Mode Control”. With the conventional burst mode control methods, output regulation is provided by short intervals of narrow duty cycle switching pulses (i.e., “burst intervals”) at low load or no load conditions. These burst intervals typically are configured to start once the output voltage drops below a lower value and to stop once the output rises above an upper value. Thus, the time at which the burst intervals start and stop with respect to the ac input voltage is random. This random starting and stopping of the burst intervals with respect to the ac input voltage may result in higher switching losses, lower efficiency, as well as increased EMI noise. In some cases, audible noise may even result.